The poachers had used an AK-47 rifle to kill the adult female rhino, the official said, adding with the killing of this rhino, the toll rose to three this year.
The KNP security personnel heard gunshots and launched a search operation, and they found the carcass near Tunikati forest camp, KNP Divisional Forest Officer Ruhini Ballab Saikia told PTI.
According to Saikia, altogether 27 empty cartridges of AK-47 rifle were recovered from the site.
Poachers killed two rhinos earlier this year. While one was killed at Hatibali Chapori in KNP's sixth addition area in February, the other was killed at Burapahar Range of the world heritage site in April, a forest official said.
Last year, poachers killed 18 rhinos in park and took away their horns, the official said.